1. Field of the Invention
The present invention relates to an apparatus for communication devices, in particular an apparatus for providing a line status of a phone line.
2. Description of the Related Art
Modems are communications devices that employ digital processing techniques to transmit data over a band-limited communications channel, such as the Public Switched Telephone Network (PSTN). Generally, to transmit data over the PSTN, a modem modulates the binary data and then transmits the data over the band-limited communications channel. A receiving modem is coupled to the band-limited communications channel, and receives the binary data and demodulates the binary data.
The modem can also be coupled to a private network. The private network can operate as a stand-alone network or be further coupled to other networks, such as the PSTN. The private network, such as a Public Branch Exchange (PBX) system can include many users, such as telephones, facsimile machines, etc. When the modem needs to initially send or receive data, the modem needs to go to an xe2x80x9coff-hookxe2x80x9d state.
xe2x80x9cOff-hookxe2x80x9d refers to a state of the communication device, such as when the communication link between the device and the PSTN is enabled for voice, data communication or network signaling. The term xe2x80x9coff-hookxe2x80x9d is derived from the original telephone usage in which they refer to the position of the hand set with respect to the cradle of the telephone.
Sometimes the modem and other communication devices such as a telephone (or an extension), facsimile machine, can share a single telephone line. Typically, should the modem need to seize the telephone line, the modem goes off hook. An off hook relay closes coupling the modem to the telephone line. The off hook relay is typically a mechanical relay that adds noise to the line when it opens or closes. Thus, should other communication devices be on line, and the modem goes off hook, the noise added to the line may affect the other communication devices that are currently using the line.
Some modem designs have incorporated opto-couplers, transistors, and capacitors to ascertain the status of the telephone line. However, the addition of these components can drive up the cost of the modem. Furthermore, the communication devices coupled to the PSTN need to meet certain government standards. The Federal Communication Commission (FCC) requires communication devices that are coupled to the PSTN meet certain electrical requirements, which include electrical interface specifications and electromagnetic interference (EMI) requirements. Furthermore, the additional hardware must be designed to meet these requirements which in turn may drive up the cost of the modem, require additional development and testing, and delay entry of the device into the marketplace. Thus, it would not be advantageous to add additional hardware to the modem.
Briefly, a method and apparatus is disclosed that provides a status of a telephone line for a communication device, such as a modem that may be shared by other communication devices, such as telephone extension, facsimile machine, etc. The apparatus provides the necessary hardware and software for detecting the line status for the communication device. With the apparatus ascertaining the state of the line, the modem can avoid going off hook when the line is already active. This can in turn prevent the noise generated when going off hook from interfering with the operation of other communication devices coupled to the same line.
An energy level in a caller ID capacitor is monitored by a modem""s data pump. Typically, the caller ID capacitor is always coupled to the line, regardless of whether the modem is in an on-hook or off-hook state. In addition, typically the energy level of the caller ID capacitor is very low if none of the other communication devices are transmitting on the line. Should other communication devices be transmitting information on the line, the AC component of the transmission can be detected from the energy level of the caller ID capacitor. The energy level is monitored by the modem""s data pump and should this energy level exceed a certain threshold, the modem will determine that the line is in use and not go to the off-hook state.
In another implementation, the modem""s data pump monitors the energy level of the telephone line. An energy profile is established for the other communication devices when they transition from an on-hook state to an off hook state and vise versa. When the data pump detects these transitions, the data pump can report the state of these communication devices or report whether the telephone line is in use or not in use.
In another implementation, should the modem need to go off-hook, a timer periodically checks the energy level of the caller ID capacitor. If the timer has not timed out and the energy level of the caller ID capacitor continues to exceed the certain threshold, a command is sent to the modem to avoid going off-hook. However, during the time period, should the energy level of the caller ID capacitor go below the certain threshold, the modem will go off-hook and begin data transmission or reception.